1. Field of the invention
The invention pertains to a process and a corresponding device for supplying current to an electric-arc melting unit for melting and heating metal, especially steel, which unit is operated with at least two electrodes.
2. Description of the Related Art
Metals, especially steel, are usually melted and heated in melting units by means of arcs. These electrically operated melting units, especially arc furnaces, are operated with either direct current, alternating current, or three-phase alternating current. At least one electrode, which passes through the furnace cover and projects into the furnace vessel, is usually used, whereas the other electrodes are either installed like the first or installed in the bottom of the melting vessel.
An arc furnace for the melting and refining of metal, especially steel scrap, is known from DE-OS 2,510,326. The furnace can be operated with a direct-current source and has at least one electrode of a certain polarity. The electrode is mounted in the treatment vessel so that it can make contact with the charge. The furnace also has a number of other electrodes of the opposite polarity, which project into the vessel above the charge. The required electric current is supplied to this known furnace by way of a star-delta-connected transformer. The various phases of the secondary windings of the transformer are connected separately to the phase inputs of a full-wave rectifier.
This known system for supplying the arc with current provides no devices for preventing feedback effects on the mains, nor can the electric power be adjusted to a desired value to melt the charge.
A direct-current electric-arc furnace with an electrode designed as a cathode projecting into the furnace vessel and at least two bottom electrodes is known from DE 4,118,756 C2, where the electrodes are connected by current-carrying lines to controllable rectifiers. At least two sets of 6-pole thyristors are used as rectifiers.
The object of the above-cited document is an attempt to solve the problem of how to influence the arc in a direct-current arc furnace. The disadvantages inherent in the proposed system such as the undesirably high mains feedback effects, poor electrical efficiency, and limited system availability are accepted in return for the ability to control the arc.
A device for supplying a melting unit with direct current is known from DE 195-36,545. This device represents an attempt to solve the problem of how to reduce the mains feedback effects of thyristor control elements. It is known from this document that a certain effect can be exerted on the feedback by controlling the firing angles, but the mains feedback effects which occur can be decreased to only a limited extent. This solution also suffers from the disadvantage that the melting capacity of the system is limited after the failure of one or more components. In addition, the selected type of asymmetrically controlled firing angles for the thyristor control elements leads to interharmonic ripple currents, which represent an undesirable load on the mains.
A device is known from EP 0,429,774 A1, which focuses on the problem of reducing the mains feedback effects of three-phase a.c. furnaces. For this purpose, a controllable choke is proposed, which is connected to the input circuit of the current-supplying three-phase transformer.
The object according to this document, however, can reduce the feedback to the mains only to a limited extent, because the power capacity of the choke connected to the input side has an effect on the process technology behavior of the arc. Thus, additional equipment is usually required to reduce the mains feedback effects to the required extent.
The goal of the invention is to create a process and a corresponding device for supplying current to an electric arc for the melting of metal, especially steel, in which, by the use of a simple design, mains feedback effects are reduced to a minimum, the electrical efficiency is increased, and the service life of the working materials of the melting unit is simultaneously prolonged.
According to the invention, the electric current from the mains to a three-phase a.c. source is divided overeat least two power supply modules connected in parallel to each other.
Each of the individual power supply modules has an uncontrolled three-phase bridge, a direct-current intermediate circuit, and a transistor unit connected in series in the current flow direction. In each branch circuit, the current is introduced as direct current by the three-phase bridge and stored temporarily in the individual direct-current intermediate circuits. The rectified partial currents are then controlled individually by the transistor units and sent via current-carrying lines to the individual electrodes and back again.
The storage capacities of the individual direct-current intermediate circuits are designed to compensate for the voltage variations arising as a result of the operation of the arc or arcs. This ensures that the load imposed on the mains power supply remains uniform. The individual current modules are designed in such a way and provided in such number that the reactive power uptake of the power supply unit can be kept constant.
In addition, the number of power supply module is selected so that, when there is a problem with the working materials, the process can be continued without interruption or limitation of the required power while certain specific modules or components are removed.
In an advantageous design, the transistor units of each power supply module are provided with components by means of which the level of the current and its form can be controlled in correspondence with the power required for the melting process while the power factor is kept constant at the same time.
In addition, each of the transistor units is connected to a control unit, by means of which the type of current can be selected. It is possible according to the invention to use the proposed power supply to generate direct current, single-phase alternating current, or three-phase alternating current.
The melting unit connected via current-carrying lines to the power supply has electrodes appropriate to the type of current, which electrodes either project down into the furnace vessel from above or are designed as bottom electrodes.
The power supply modules can be connected individually or in groups to phase-shifted, three-phase power sources. This improves the feedback effects on the mains by reducing the ripple currents resulting from the characteristic of an nxc3x976-pulse circuit design.